The present invention relates to a mobile communication system having a base station and a plurality of mobile stations, and in particular to a mobile communication system having a direct communication mode between mobile stations and a communication method therefor.
In the field concerning business radio communication in Japan, introduction of a narrow band digital radio communication technique using the SCPC (Single Channel Per Carrier) scheme is advanced in recent years for the purpose of using radio frequencies efficiently. The SCPC system is described in “Narrow Band Digital Telecommunication System (SCPC/FDMA)”. ARIB STD-T61, published by Association of Radio Industries and Business, May 1999, pages 3, 5, 14, 16, 35, 36, 37 and 40.
Therefore, an example of a mobile communication system using the narrow band digital radio communication technique will now be described roughly. In the system, an operational area (zone) indicated by a broken line is first formed by constructing a base station (radio equipment) having, for example, a command console or an operator console (equipment) as shown in FIG. 6.
And, by the construction of the operational area, operation of mobile communication among a plurality of mobile stations (terminals), such as, for example, A, B, C, D and E as illustrated is made possible therein. As for this system, operation using two kinds of, i.e., first and second communication modes described below are conventionally known.
In the first communication mode, radio carriers f1 and f2 serving as a pair of up-link and down-link carriers are assigned to the base station. By using these up-link and down-link carriers, communication is conducted between the base station and a mobile station, or between a mobile station and another mobile station via the base station with the press-to-talk scheme. Since operation is conducted under the control of the command console in both cases, the first communication mode is hereafter referred to as base station dependent mode.
By the way, the press-to-talk scheme is one communication scheme of radio machines, and refers to a scheme in which transmission is conducted by using one transmission line alternately in a time division manner (simplex scheme). Usually the radio machine is set to the reception state. When transmission is desired, a press button of the radio machine is pressed to conduct transmission. Therefore, it is called press-to-talk scheme. A plurality of radio machines share one transmission (frequency channel). Once one radio machine starts transmission line and occupies the transmission line (this state is called busy), other radio machines cannot conduct transmission until the talk is finished.
In the second communication mode, both mobile stations use a carrier f3 for direct communication, and communication is conducted between the mobile stations without interposition of the base station. Communication mode switching in the mobile station is typically conducted manually. Hereafter, it is referred to as direct communication mode.
In such a system, however, the base station dependent mode is typically the original communication mode, and this mode is the basis. In many cases, therefore, the direct communication mode is added as an option.
Referring back to FIG. 6, the radio carrier f1 serving as a transmission wave of the base station (or a reception wave of a mobile station) is called down-link carrier, and the radio carrier f2 serving as a transmission wave of the mobile station (or a reception wave of the base station) is called up-link carrier. The up-link carrier and the down-link carrier form a pair.
As for frequency relations of the radio carriers with the up-link carrier and the down-link carrier paired off, paired carriers f1 and f2 related by an offset frequency Δf are selected respectively from a down-link band F1 and an up-link band F2 as shown in FIG. 7. On the other hand, as for the carrier f3 for direct communication, one carrier included in the up-link band F2, which is not used by the base station, is selected.